This section shows two MPPT control performances in solar system, which are perturb & observe and sliding mode extremum seeking control. The tracking effects of them are also compared.
5.2.1 Perturb and Observe Control
Similar with wind P&O control, the solar P&O control is also separated into linear load and non-linear load[1]. Simulation of PV P&O profile for a 30Ω load when the temperature is (25◦C) has been shown in Figure 5.12[1]. Figure 5.12(a) depicts the PV current Ipv is around 17.1 A under the standard weather condition 1000W/m2[1].
This shows the reach of maximum power point delivered from PV panel[1]. Figure 5.12(b) illustrates the variation of perturbation signal during the irradiance changing
from 600W/m2 to 1000W/m2 and then come back to 600W/m2, which oscillates
slightly around the best duty ratio when the radiation changes quickly[1]. As the figure shows, the maximum power points of PV model are approximately 1010W, 1400W and 1800W when the solar radiances are 600W/m2, 800W/m2 and 1000W/m2
respectively[1].
In order to evaluate the MPPT tracking efficiency for solar system, Table 5.2 demonstrates the power condition under different irradiance levels[1]. From the table, the output power generated from PV cells, the load power and the power loss are calculated individually[1]. The results show that the tracking efficiencies are more than 97% generally under the step change from 600W/m2 to 1000W/m2 to 600W/m2.
(a) PV current profile
(b) MPPT tracking effect
Fig. 5.12. PV P &O MPPT Profile to step change in the irradiance from 600W/m2
Table 5.2 MPPT Tracking Efficiency in Solar System
SolarIrradiance P V P ower LoadP ower T rackingEf f iciency
600W/m2 1000W 980W 98%
800W/m2 1400W 1360W 97.1%
1000W/m2 1800W 1750W 97.1%
When the load is a DC constant power load (1.5kW)[1]. Simulations of solar MPPT profile for a DC constant power load (1.5kw) and battery with variations of illumination are illustrated in Figure 5.13 and 5.14[1]. Figure 5.13 (a) and (b) show the solar output current and power tracking when the illumination changes from 800W/m2 to 1000W/m2. Figure 5.14 (a) and (b) depict the solar output current and
(a) PV current profile
(b) MPPT tracking effect
Fig. 5.13. PV P &O MPPT Profile to step change in the irradiance from 800w/m2 to 1000w/m2 with CPL (1.5kw)
(a) PV current profile
(b) MPPT tracking effect
Fig. 5.14. PV P &O MPPT Profile to step change in the irradiance from 1000W/m2
5.2.2 Sliding Mode Extremum Seeking Control
In order to verify the sliding mode extremum theory, the whole system is built in Matlab/Simulink[2]. The validity of the proposed system has been checked in the following figures, the sample time of the simulations has been set to 1e-6 sec[2]. The g1 conductance of the first LFR with the illumination changing from 1000W/m2 to
800W/m2 is shown in Figure 5.15. The g
2 conductance of the second LFR with the
illumination changing from 700W/m2 to 900W/m2 is illustrated in Figure 5.16. It is clear that the two conductance values can be controlled automatically under the variations of solar radiations. The waveforms for the cascaded converters behaving as LFRs supplied from PV source under MPPT by applying sliding mode control depicted in Figure 5.17[2]. Figure 5.17 (a) shows the MPP tracking profile with an irradiance change from 1000W/m2 to 800W/m2 and Figure 5.17 (b) illustrates
the waveforms of PV power, voltage and current and first stage voltage during an irradiance change from 800W/m2 to 600W/m2[2].
Fig. 5.16. g2 value under Irradiance changing from 700W/m2 to 900W/m2
Then Figures 5.18 and 5.19 represent the system responses under a step change of the load DC voltage from 240V to 380V and 380V to 240V to show the feasibility of the control system[2]. It can be observed that increasing or decreasing the output voltage does not affect to capture the maximum power of the solar panel in the proposed system[2].
(a) MPPT tracking profile with illumination changing from 1000W/m2to 800W/m2
(b) PV power, voltage, current and first stage voltage with illumination changing from
800W/m2 to 600W/m2
Fig. 5.17. Waveforms for the cascaded converters behaving as LFRs supplied from PV source under MPPT by applying sliding mode control at fixed temperature (25◦C)
Fig. 5.18. Waveforms of the PV power, PV voltage and current during the load voltage changes from 240V to 380V
Fig. 5.19. Waveforms of the PV power, PV voltage and current during the load
5.2.3 Comparisons
The MPPT dynamic response of P&O and SM-ESC algorithms with constant solar irradiance are compared in Figure 5.20. From the comparison results, regardless of the 9W power error in solar system, it can be observed that the output power oscillations of SM-ESC method in solar system are smaller than the P&O method, and the tracking speed is nearly the same. The 9W power error in SM-ESC is acceptable with respect to the total generating power around 1300w. Therefore, the SM-ESC control strategy can definitely enhance the capturing efficiency and has better tracking performance than the traditional P&O method.